82 Cincinnati Society of Natural History. 



alis, aud from the great bend of the Missouri river below Fort Pierre, 

 Ostrea inornata. 



From (Niobrara Group) near the mouth of the Niobrara river, An- 

 omia ohliqua ; from (Dakota Group) near the mouth of the Big 

 Sioux river, 3Iactra Siouxensis. F. B. Meek described, from near 

 Bear river, on Sulphur Creek, Anomia concentrica, Corhula concentri- 

 ca, C. engelmanni, C.pyriformis^ and 3Ielania humerosa, now Pyrgu- 

 lifera humerosa; from the North Platte, Inoceramus simpsoni ; from 

 Ham's Fork, northeast of Fort Bridger, Melampus priscus, now Rhy- 

 tophorus priscus, Melania simpsoni^ now Goniohasis simpsoni, M.- 

 arcta, M. nitidula, now Limncea nitidula^ L. similis^ L. vetusta, 

 Planorhis spectabiUs^ P. utahensis, and from near Fort Bridger, Uiiio 

 haydeni. Some of the latter species probably belong to the Lower 

 Eocene. 



In 1861, Meek and Hayden,* as before mentioned, separated the 

 Cretaceous rocks of the Missouri region into five subdivisions, in 

 ascending order, as follows : 



1. Dakota Group, consisting of yellowish, reddish, and occasionally 

 white sandstone, with, at places, alternations of various colored clays 

 and beds and seams of impure lignite ; also silicified wood, and great 

 numbers of leaves of the higher types of dicotyledonous trees, with 

 casts of Pharella dakotensis, Axinaea siouxensis, and Cyprina arenarea. 

 Found at the hills back ot the town of Dakota; also extensivel}' de- 

 veloped in the surrounding country in Dakota county, below the mouth 

 of Big Sioux river, thence extending southward into northeastern 

 Kansas and be3^ond. Estimated thickness, 400 feet. 



2. Fort Benton Group, consisting of dark gra}', laminated cla3^s, 

 sometimes alternating near the upper part with seams and la3^ers of 

 soft gray and light colored limestone, Inoceramus prohlematicus, I. 

 tenuirostratus, I. latus, I,fragilis, Ostrea congesta, Venilia mortoni, 

 Pholadomya papyracea, Ammonites mullani^ A. percarinatus, A. ves- 

 pertinuSf Scaphites loarroii, S. larvcuj-ormis, S. ventricosits, S. vermi- 

 formis, Nautilus elegans, etc. Extensively developed near Fort Ben- 

 ton, on the Upper Missouri; also along the latter from ten miles above 

 James river to Big Sioux river, and along the eastern slope of the 

 Rock}^ Mountains as well as at the. Black Hills. Estimated thickness, 

 800 feet. 



3. Niobrara Group, consisting of lead-gray calcareous marl, weather- 

 ing to a yellowish or whitish chalk}^ appearance above, containing 



■•'■ Proe. Acad. Nat. Sei. 



